1. Field of the Invention
The present invention relates to a hydraulic bearing device that is mounted, for example, on a main spindle of a machine tool.
2. Description of the Related Art
As illustrated in FIG. 1, a previously employed hydraulic bearing device to be mounted on a main spindle of a machine tool or the like is provided with a bearing metal 30. Referring to rig. 1, a substantially U-shaped groove 31 is formed in a bearing surface of the bearing metal 30 which supports a rotating shaft such as the main spindle of the machine tool. An oil supply hole 8 having a throttle nozzle opens to a bottom surface of the substantially U-shaped groove 31. The substantially U-shaped groove 31 is composed of a connecting portion 31a that extends in an axial direction of the rotating shaft, and log portions 31b that extend from respective ends of the connecting portion 31a in the circumferential direction in which the rotating shaft rotates. Drainage grooves 32 extending parallel to the leg portions 31b are formed in the bearing metal 30, so that the substantially U-shaped groove 31 is interposed between the drainage grooves 32. A quadrilateral bearing surface area surrounded by the substantially U-shaped groove 31 constitutes a dynamic pressure generation land 6.
In the aforementioned bearing metal 30, lubricating oil in the oil supply hole 8 undergoes a pressure reduction process provided by the throttle nozzle and flows into the substantially U-shaped groove 31, thus filling up a space defined by the substantially U-shaped groove 31 and an outer peripheral surface of the rotating shaft. The lubricating oil then flows through a narrow gap formed between a bearing surface area and the outer peripheral surface of the rotating shaft into the drainage grooves 32 and is drained to the outside. Therefore, the hydraulic bearing device with the bearing metal 30 functions as a static fluid bearing.
Since the diameter of the rotating shaft is smaller than the inner diameter of the bearing metal 30 having the dynamic pressure generation land 6, a bearing clearance is formed between a bearing surface of the dynamic pressure generation land 6 and the outer peripheral surface of the rotating shaft. The lubricating oil that has filled up the space defined by the substantially U-shaped groove 31 and the outer peripheral surface of the rotating shaft remains in the bearing clearance. When the rotating shaft rotates, a dynamic pressure is generated due to a wedge effect of the lubricating oil remaining in the bearing clearance formed between the bearing surface of the dynamic pressure generation land 6 and the outer peripheral surface of the rotating shaft. A a result, the hydraulic bearing device with the bearing metal 30 also functions as a dynamic fluid bearing.
Recently, machine tools adopt a main spindle that rotates at a high speed. In the aforementioned previously employed hydraulic bearing device, if the rotating shaft rotates at a high speed, the dynamic pressure generation land 6 is subjected to a great heat generated by the lubricating oil due to fluid friction. Consequently, the bearing metal 30 thermally expands. Because an outer side of the bearing metal 30 is fixed, the inner diameter of the bearing metal 30 decreases and the bearing clearance formed the dynamic pressure generation land 6 and the rotating shaft decreases correspondingly due to the thermal expansion. As a result, the dynamic pressure generation land 6 is further subjected to a greater heat generated by the lubricating oil.
Then, the bearing metal 30 thermally expands further and the bearing clearance decreases, so that the lubricating oil generates an even greater heat. This results in a vicious circle.
Continuation of such a vicious circle leads to deterioration of bearing performance and possible seizure of the bearing metal 30 with the rotating shaft.
In order to prevent deterioration of the bearing performance as well as seizure of the bearing metal 30, the surface area of the dynamic pressure generation land 6 may be reduced or the bearing clearance may be increased. In this case, however, the dynamic pressure applied to the lubricating oil by the dynamic pressure generation land 6 decreases, so that bearing load characteristics may be adversely affected.
In the case where the rotating shaft rotates at a comparatively high speed, the demand for an increase in dynamic pressure supporting rigidity as well an dynamic pressure conflicts with the demand for a decrease in excessive heat generation in the vicinity of the dynamic pressure generation land 6.